Does Second To Fourth Digit Ratio Predict You-Know-What Length?

From the storied Halls of Science comes an important new study, here abstracted from the Asian Journal of Andrology1. I apologize for the length of this excerpt, but it is necessary you read it all (viaPubMed; spaces entered for readability):

The second to fourth digit ratio (2D:4D) has been proposed as a putative biomarker for prenatal testosterone and covaries with the sensitivity of the androgen receptor (AR). Both prenatal testosterone and the AR play a central role in penile growth. In this study, we investigated the relationship between digit ratio and penile length.

Korean men who were hospitalized for urological surgery at a single tertiary academic centre were examined in this study, and 144 men aged 20 years or older who gave informed consent were prospectively enrolled. Right-hand second- and fourth-digit lengths were measured by a single investigator prior to measurement of penile length. Under anaesthesia, flaccid and stretched penile lengths were measured by another investigator who did not measure nor have any the information regarding the digit lengths.

Univariate and multivariate analysis using linear regression models showed that only height was a significant predictive factor for flaccid penile length (univariate analysis: r=0.185, P=0.026; multivariate analysis: r=0.172, P=0.038) and that only digit ratio was a significant predictive factor for stretched penile length (univariate analysis:r=-0.216, P=0.009; multivariate analysis: r=-0.201, P=0.024; stretched penile length=-9.201 x digit ratio + 20.577).

Based on this evidence, we suggest that the digit ratio can predict adult penile size and that the effects of prenatal testosterone may in part explain the differences in adult penile length.

I can shoot a deer without flinching and can gut a fish faster than you can blink. I have even unconcernedly eaten raw frog gall bladders (bitter) and bags of marinated duck tongue. I can walk through an emergency room with complete indifference and am not repulsed by changing diapers of colicky babies.

But I am otherwise squeamish and don’t have the courage to investigate how these researchers got to the “stretched” in “stretched penile length.” I’ll can only assume the method, performed “under anaesthesia”, was charitable and caused no lasting damage.

As far as “flaccid” goes, we have no information on the temperature of the operating room, but let’s hope it remained constant over the duration of the experiment.

In any case, these researchers have given us a new method to predict the length of a man’s certainty without requiring him to drop his shorts. The equation, helpfully given to the third decimal place, and expressed in centimeters, is:

    stretched penile length=-9.201 x digit ratio + 20.577.

For those unwilling or unable to do the math, I have prepared this helpful chart. You must first measure the length of your right hand’s forefinger and then divide into this the length of your ring finger. That is the “Second to Fourth Digit Ratio.” Find that ratio and head straight up until you intersect the sloping line. Once there, find the number parallel to it on the left, or y-axis.

forefinger to ring finger

I have done this for a ratio of 1 (or 1 to 1). The model predicts that men whose fore- and ring-fingers are the same length have a stretched penile length of 4.5 inches. That is stretched, mind: and recall we have no clear idea what stretched means.

But let’s notice something curious about this graph. I was in the military and, as a consequence, have seen a host of men in, um, various positions of authority (I was Air Force; Navy men will have more knowledge than I). By this, I mean I can report to you that many men are to be found on the upper regions of the y-axis.

According to this model, then, a man of about 7 inches, a far from unheard of territory, is predicted by a digit ratio of 0.2. So if such men have forefingers of 3.25 inches (my measurement), their ring fingers must be 16.25 inches (because 3.25/16.26 = 0.2). My dears, this is one foot, four-and-a-quarter inches. Coney Island itself has never been witness to such freakishness!

What this means is that either this model is a dismal statistical failure, or that Korean men differ substantially from American men, in the direction of being severely shortchanged by Nature or because they are being adversely affected by a steady diet of kimchi. Though I have been to Korea twice, I had no opportunity to take field notes on this consequential topic, so I cannot say which theory is true.

Left unanswered is the most important question of this study: why? I’ll leave that for you to guess.


1Besides the Asian Journal of Andrology, there is the just plain Journal of Andrology, the (exclusive of Asians?) International Journal of Andrology, the (non Asian?) Chinese Journal of Andrology.


  1. a man of about 7 inches, a far from unheard of territory, is predicted by a digit ratio of 0.2. So if such men have forefingers of 3.25 inches (my measurement), their ring fingers must be 16.25 inches

    Perhaps you are reading too much into this and the model is only useful in the 0.8-1.2 (or some other) ratio range? Not specifying limits is a common fault.

    lengths were measured by another investigator who did not measure nor have any the information regarding the digit lengths.

    They didn’t/couldn’t even sneak a peak at the hands while doing this?

  2. FRom a statistics standpoint, don’t the very low r values indicate very little predictive power (ie. relatively little correlation) despite the “good” p values? In other words, isn’t this statistical hogwash?

    As to the subject of the study, I think just because you can study something doesn’t mean you should study something.

  3. Another instance of P-value (!) worship, eh?

    It amazes me that they can report stuff like, r ~= 0.2, and then say, “significant predictive factor,” in the same context. Well, sure, significant in predicting about 4% of the variation in the dependent variable!

    One of the clearest lessons remaining from my study of regression is about r-squared. Who cares about P values if you’re not accounting for any of the variation?

  4. DAV,

    It’s true the model may be useful in only a range, but that range would appear to be extraordinarily narrow. Even a ratio of 0.8 predicts a ring finger length of 4.1 inches, which is almost a full inch longer than the forefinger. And as any woman—or man—will tell you, an extra inch is a lot. Plus, it does fail to account realistically for men larger than about 5 inches.


    The p-value is small, so the model must be true. Or if not true, at least publishable.

  5. I bet if you plotted the raw data points on that graph, they’d form a tiny blob right around x=.95.

    From now on, this should be your example for how statistical significance assumes a correct model.

  6. I read the post with a kind of primitive, focused, fascination – actually there was a kind of cold, private, consideration I experienced. Until I realized I didn’t have my glasses on and I thought it read “Asian Journal of Astrology”.

    Oh. Andrology.

    duh. Now it all makes perfect sense.

  7. If you calculate the formula in NASA style, you can confuse inches with centimeters. That way you can meet spec.

  8. It’s clear from the article that this is flaccid stretched length, and has nothing to do with the size of the penis when erect.

  9. ad,

    I invite you to re-read the sentence which ends, “stretched penile length=-9.201 x digit ratio + 20.577″, emphasis mine.

  10. John, thank you ever so much. Up until I read your comment, the most I was worried about was a colonoscopy.

  11. This explains how those “male enhancement” products work: they shrink the forefinger. Science is amazing!

  12. I’m having trouble reconciling your assertion that you can tell us something about the distribution of men on the y-axis, particularly that 7 inches is far from unheard of, and your repeated reminder that we don’t know what stretched means.

    Combining your implication with the reports of the study that say the average stretched length was 11.7cm (4.6 inches), it is hardly unlikely that either this group of Koreans needing urological surgery are indeed different to your fellow USAF members or you are tlking about different measurements, possibly inevitably.

  13. Jonathan,

    I was joking about not knowing what “stretched” means. Next time I’ll put an animated eyebrow waggle next to my masterly puns to signify funny times are here!

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